In various applications beginning with back-up power sources and the like for communication apparatuses, plural secondary batteries are serially connected and used as assembled batteries. However, even just after manufacture of secondary batteries, variations exist in the properties of the individual batteries. Such variations grow as the duration of service lengthens and as battery deterioration progresses, and are manifested in voltage disparities or the like.
In recent years, demands have grown for increasingly compact and lightweight secondary batteries in power systems, and there is a trend toward the use of lithium ion batteries with high energy density. These lithium ion secondary batteries are used with settings where the charging voltage per lithium ion battery is 4.1V to 4.2V, and the discharge cut-off voltage is 2.9V to 3.0V. This is because, when charging is performed at high voltage and discharge is performed at low voltage, the electrode material and electrolytes forming the secondary battery undergo chemical reaction, and secondary-battery performance declines. Accordingly, it is necessary to strictly set the discharge cut-off voltage and charge cut-off voltage of lithium ion secondary batteries. However, with lithium ion secondary batteries, the phenomenon is observed in which variations in the terminal voltages of the respective batteries tend to occur when serially connected assembled batteries are constituted, and that these voltage variations gradually grow once the variations have occurred. As this phenomenon progresses, it greatly affects on the life of each battery and the discharge performance of the assembled battery.
On the other hand, when assembled batteries are incorporated into power systems and used, it is necessary to maintain the capacity of the assembled battery, and various charging methods are adopted that take into account the properties of the secondary batteries and the configuration of the power system. In direct-current power sources for communications, lead-acid secondary batteries are primarily used, and the constant-current constant-voltage charging method is adopted. In this method, the load and the secondary battery are connected in parallel to the rectifier output. Consequently, it is possible to instantaneously switch secondary battery discharge at times of rectifier malfunction or power outage of commercial power sources. Additionally, after recovery from a power outage, there are the advantages that it is possible to supply power to the load while performing storage-battery charging, and that it is basically possible to perform capacity maintenance of the secondary battery by regulating the output voltage of the rectifier.
This constant-current constant-voltage charging method is also suited to the charging of lithium ion secondary batteries, and this battery is considered to be suited to communications applications from the standpoint of the charging method. When lithium ion secondary batteries are serially connected and used, well balanced charging can be performed if the capacities or internal resistances of all batteries are constantly identical. Yet, in reality, there exist slight variations in the capacities or internal resistances of batteries. Furthermore, even if internal properties are initially identical, the internal properties of batteries are changed by trickle charging or float charging as time passes. Consequently, in the charging of conventional secondary batteries, the method has been adopted where means of measuring individual battery voltage are provided, warning signals are emitted when battery voltage exceeds a prescribed value, charging or discharging is prohibited, for example. Yet, with this method, there is the major drawback that the progress of charging is impeded, and battery performance cannot be fully realized.
Additionally, in assembled batteries maintained by the constant-current constant-voltage charging method, it is also conceivable that parts may be attached for suppressing the cell voltage of each battery. However, merely by the simple attachment of a cell voltage suppression part, differences will occur in the charged state of each battery when performing recuperative charging after discharge if the internal states of the respective batteries differ. As a result, under circumstances where a constant charging current is flowing, there are the problems that even if the charging of a certain battery is complete, other batteries are in the process of being charged, that the bypass current for voltage suppression is a high value, that part size is large, that the price is high, for example. Thus, this method has not reached the stage of practical use.
The present applicant has previously filed the description of Japanese Unexamined Patent Application, First Publication No. 2003-157908, as a charging device for charging assembled batteries.